A conventional method of constructing a structure on the sea is roughly classified into two. One is a creation of a land on the sea either by "reclamation" or by "reclamation by drainage" followed by an ordinary construction of a structure on the land. The other is a floatation of a structure on the sea. The reclamation method is that an embankment is constructed along the periphery of an expected reclaimed land, which sea-bottom poor subsoil at the area to be reclaimed is improved, and a strucuture is constructed on the ground prepared by filling up an embanked area with earth and sand or the like. The method of land reclamation by drainage is that a bank is constructed in the shallow water offshore, water is drained from an embanked area to expose the ground at sea-bottom, and a construction of a structure is executed similarly to the reclamation method. The flotation method is that a floating structure is allowed to simply keep afloat on the sea due to buoyancy, while being moored to the bottom of the sea.
Among the above methods, the construction of a structure by the reclamation method is accomplished through process of execution of works similar to the construction of a normal land structure, after the reclaimed land has been created and improved if necessary. Therefore, the structure in this case, when accomplished, is resistant to wind, tidal current or the like as the land structure. On the other hand, a lot of time and cost have been required to suffer a great loss in a term of works and a cost of construction, until a start is made with construction of a structure. Besides, there is a possibility of encountering the settlement of ground with the passage of time or a liquefaction risk when an earthquake happened. It should be noted that, within the context of the present specification, terms "settling" and "settlement" refer to subsidence or sinking of the land at the water bottom due to the weight of the structure. Further, within the context of the specification, terms "improve" or "improvement" of land refers to procedures on the land at the water bottom to increase the strength thereof against the weight of the structure. Further, when a place adjoining the already-reclaimed land is reclaimed, the already-reclaimed land is dragged toward a newly-reclaimed land. Thus, the existing facilities on the already-reclaimed land are liable to differential settlement. As a result, it is greatly difficult to enlarge the plan of reclamation according to the reclamation method.
According to the method of a land reclamation by drainage, since the reclaimed ground is on a level with or below the surface of the sea, the safety of a structure against disaster is dependent on the reliability of the bank. However, when the bank is broken due to an earthquake or storm surge, the structure is left in a defenseless state. Further, a long period of time should be spared for the construction of bank and drainage of water as preliminary works for the construction of the structure, similarly to the reclamation method.
The structure according to the flotation method is separated from the sea-bottom ground, and hence, is not directly affected by seismic force, nor settles. Thus, this structure is highly secured against earthquake damage. However, since the structure is afloat on the sea it is easily subjected to rocking motion due to wind or tidal current. Such a floating structure has a defect in stability, and in the worst case, is in danger of drifting, sinking or capsizing. Further, when the area of the structure is enlarged, the stiffness of the structure is relatively reduced. However, as long as the structure is afloat on the sea, the floating structure is always affected by the tidal current, and is liable to partially disordered motion. As a result, it is not possible to attain a structure of a substantially large plane area.
As described above, the conventional methods other than the flotation method inevitably require a long period of time until the completion of the structure, and pose various problems including points on preservation of environment. Namely, a construction field cannot be restored to the former condition, after the construction work has been accomplished.
In view of the background art described above, the present applicant has proposed a structure and an installation method thereof in Japanese Patent Laid-open No. 4-85410 in order to overcome the weak points of the conventional methods. According to this proposal, the contact pressure of a structure is set to the desired degree by regulating the own weight of the structure with water serving as ballast, and the structure thus ballasted is installed on the excavated bottom of the sea, whereby the structure lands on the bottom of the sea without floating up to the surface, nor settling, and maintains the stable state so as to be resistant to horizontal force caused by an earthquake, waves and tidal current or the like. Therefore, the structure and the installation method thereof thus proposed have the advantages of not only reducing a term of works and a cost of construction, but also attaining high safety and stability on the sea. However, the structure is installed without settling or maintains the stability to be resistant to horizontal force in dependence on only the effect of excavation of the ground. Thus, a considerable range of the bottom of the sea should be excavated in accordance with the ground conditions, or other conditions such as the range of variation in the water level and the magnitude of wave pressure, and in the worst case, the execution of works itself fails to realize.
Further, the horizontal stability of the structure in the floating state under tugging or construction and that of the accomplished structure in the landing state can be ensured by regulating the quantity of water in each ballast tank. However, when only a pair of ballast tanks are provided in one direction, it is difficult to ensure the stability of the structure in the direction orthogonal to the ballast tanks.